Abradable seals are used on the shrouds of compressors, particularly gas turbine engines of jets or land based rotary machinery, to assure efficient operation of the engines by minimizing gas leakage in both the compressor and turbine sections of the engines. Although the engine is typically designed and manufactured to precise dimensional tolerances, thermal and centrifugal expansion of rotating and stationary members makes zero clearances difficult to achieve. Thus, abradable seals are commonly employed on surfaces of the stationary member. Abradable seals allow penetration of rotating members, thereby establishing desired gas leakage control, by creating what is effectively a low tolerance seal. Abradable seals are distinct from rub surfaces in wet environments such as exist around oil bathed friction parts in engines, couplings and brakes.
A viable abradable seal is a compromise among many mutually exclusive physical properties. The seal should wear by disintegrating into fine particles rather than by tearing or spalling, and without causing substantial wear to the tips of the rotor blade. The seal should also be highly resistant to erosion and oxidation, as the hot, high velocity gas stream passing through the engine, laden with abrasive particulate matter, creates an environment that is both highly erosive and oxidative. Various attempts have been made to provide the property of abradability in the seal by manipulating the mechanical properties of the seal. Abradability may be achieved by using low sintering temperature during the seal formation process to provide a low density seal. The mechanical strength of the seal can also be reduced by the inclusion of friable non-metallic materials such as graphite or diatomaceous earth.
Alternatively, the quality of abradability may be a function of the melting point and melting characteristics of the seal. Where the melting point of the seal is somewhat above the operating temperature of the compressor, but below the melting temperature of the rotating member, heating caused by friction at the rub surface results in melting of the seal. As the rub surface temperature approaches the melting point of the seal, the seal loses mechanical strength and is readily abraded or displaced by the rotating member. The rotating member, having a substantially higher melting point, loses little mechanical strength, and can abrade the seal without damaging the rotor tip.
Modern gas turbine engines utilize rotor blades made up of titanium alloy, operating at temperatures up to about 700.degree. C. Two main seal formulations are currently in commercial use, an AlSi-polyester seal (Metco 601--Trade Mark of Perkin-Elmer Corporation, New York) and a felt metal seal. The felt metal seals are typically formed from nickel based alloys. The seal consists of metal fibres which are sintered to produce a highly porous material (about 80% porous). The seal is applied by brazing. Neither commercially available seal is entirely satisfactory. The felt metal seal melts at temperatures substantially above 700.degree. C., too high for the conditions of the Ti alloy blades. This results in substantial blade tip wear, the seal rub surface becomes very rough, creating an aerodynamically undesirable surface, and the seal sparks during rubbing. Sparking is particularly undesirable in turbine engines utilizing Ti alloy rotor blades as the potential for a Ti fire exists. Furthermore, the low density of the seal results in a undesirable amount of leakage.
An AlSi-polyester seal creates a rough rubbing surface, sparking, and the debris of the disintegrating seal tends to stick to the rotor blades and other engine hardware. The sticking of debris to the engine hardware is aerodynamically undesirable, and the debris-air mixture can be explosive.
The prior art describes many abradable seal compositions and structures. However, none of the seals described meet the dual requirements of being suitable for use with Ti alloy rotor blades at temperatures up to 700.degree. C. and suitable for application by thermal spraying.
U.S. Pat. No. 3,053,694, U.S. Pat. No. 3,068,016 and U.S. Pat. No. 4,639,388 describe abradable seal structures in which the seal is either applied as a slurry or sintered into a metallic honeycomb matrix. Such seals cannot be deposited by thermal spraying.
U.S. Pat. No. 3,975,165, U.S. Pat. No. 3,985,513, U.S. Pat. No. 3,817,719 and U.S. Pat. No. 3,879,831 teach abradable seal compositions designed for use in the turbine sections of jet engines. Such seals are designed to withstand temperatures in excess of 1000.degree. C. These seals therefore have melting points that are too high to provide abradability under 700.degree. C. against titanium alloy blades.
Other prior art seals, such as those described in U.S. Pat. No. 5,049,450, U.S. Pat. No. 3,084,064 and U.S. Pat. No. 4,023,252 comprise compositions having melting points too low to provide seals with sufficient strength in the 345-700.degree. C. operating temperature range.
As a result of these unresolved problems, the industry has sought alternative materials, especially for use in the 345 to 700.degree. C. range. One present solution is to hard-tip the Ti alloy blades. This allows for the use of dense, hard, shroud seal coatings. However, hard tipping of the blades is expensive and can lower the fatigue life of the blades. The use of Ni alloy blades, which are harder than Ti alloy blades and need no hard tipping, is not practical as they are twice as heavy as Ti blades. Thus, a need remains for an abradable seal suitable for use with Ti alloy blades at temperatures up to about 700.degree. C. that will cause minimal blade wear, sparking or debris sticking, will be highly resistant to erosion, oxidation and roughening of the rub surface, and will disintegrate into fine particles during rubbing.